1. Field of the Invention
The invention relates to an improved piezoelectric actuator, for example for actuating a mechanical component such as a switching valve in a fuel injector system.
2. Description of the Prior Art
DE 199 28 189 A1 has disclosed that using the so-called piezoelectric effect, a piezoelectric element for controlling the needle stroke of a valve or the like can be composed of a material with a suitable crystalline structure. The application of an external electrical voltage causes a mechanical reaction of the piezoelectric element, which, depending on the crystalline structure and the regions to which the electrical voltage is applied, produces a compression or traction in a predeterminable direction.
Due to the extremely rapid and precisely controllable stroke effect, such piezoelectric actuators can be used in producing control elements, for example for actuating switching valves in fuel injection systems of motor vehicles. In these actuators, the voltage-controlled or charge-controlled deflection of the piezoelectric actuator is used to position a control valve, which in turn regulates the stroke of a nozzle needle.
Since the electrical field intensities required for actuating the piezoelectric actuator lie in a range of several kV/mm and it is generally desirable to use moderate electrical voltages for triggering purposes, the piezoelectric actuator here is comprised of a plurality of layers of stacked metallized piezoelectric ceramics to produce a so-called multilayered actuator. To accomplish this, internal electrodes that are deposited, for example, by means of a printing process are provided between the layers and external electrodes are provided to supply an electric voltage. A typical method for manufacturing such layers is the sheet casting technique. The individual layers are metallized in order to manufacture the internal electrodes and stacked on top of one another, the piezoelectric effect then exerting its action between two layers with internal electrodes of different polarities.
The top and bottom region, however, usually do not have internal electrodes since on the one hand, a certain amount of insulation space toward the end surfaces is required in order to prevent short circuits in relation to the outside and on the other hand, passive zones are used for electrical connection to the external electrodes. Passive regions can also be provided in the middle of the piezoelectric actuator. But the problem with this, however, is that these passive layers, in the form of covering packets, represent parasitic or interfering capacitances in relation to the electrical ground, which can generate electromagnetic radiation in motor vehicles or other applications.
DE 100 25 998 A1 has clearly disclosed the fact that passive regions of changeable length are provided at both ends of the layer structure. In this known piezoelectric actuator, on the one hand, the passive layers are comprised of the same ceramic material as the active region, but with external electrodes that contact only one side or do not contact them at all so that the inactive regions also have the internal electrode/metal layers passing through them. On the other hand, the respective inactive region can also be a whole electrically insulated metal or ceramic block that can simply be glued for example to the piezoelectrically active region.